Lecture 14 - Repetition - Tables, Menus & Nested Control Structures

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Objectives

To introduce nested control structures and review repetition

Displaying Tables of Values
Nested Control Structures
- Example 1: Menus
- Example 2: Membership Fees
Review of Looping
Key Points
Further Reading

1. Displaying Tables of Values

Sometimes repetition is implied by the problem rather than explicitly stated. For example:

Problem Description

Display a table of values showing the balance of a bank account with interest compounding yearly.

Example Program Run

Run
Initial balance ($) ? 1000
Interest rate (%) ? 7

Year          Interest ($)  Balance ($)
 0                          1,000.00
 1              70.00       1,070.00
 2              74.90       1,144.90
 3              80.14       1,225.04
 4              85.75       1,310.79
 5              91.76       1,402.55
 6              98.18       1,500.73
 7             105.05       1,605.78
 8             112.40       1,718.18
 9             120.27       1,838.45
 10            128.69       1,967.14

Discussion

Is this repetition?

Look for any patterns:

year always starts at 1 and goes up to 10 by 1's. This is a fixed number of repetitions (10), starting at one and increasing by 1: this can can be done using a FOR loop
The second column interest can be calculated based on the balance from the previous year. The third column balance can be calculated from the interest in that year and the balance form the previous year.

Defining Diagram

Inputs	Processing	Output
initialBalance, interestRate	Input initialBalance, interestRate Calculate year Calculate interest Calculate balance Display year, interest, balance	year, interest, balance

Outline the Solution

Pseudo Code

compoundingInterest()
    Input initialBalance, interestRate
    balance = initialBalance
    Display year, balance
    FOR year = 1 TO 10
        interest = balance * interestRate / 100
        balance = balance + interest     
        Display year, interest, balance
    ENDFOR
STOP

Data Dictionary

Name	Data Type	Description
balance	Double	The current balance of the account in $
initialBalance	Integer	The starting balance of the account in $
interest	Double	The interest earned in the current year
interestRate	Double	The interest rate payable on the account as a %
year	Integer	The current year number

Desk Check

Access Basic Code

' Name: compoundingInterest
' Purpose: Calculate the balance of an account with compounding interest
' Author: Tim Whitfort
 
Option Compare Database
Option Explicit
 
Sub Main()
 
    ' Declarations
    Dim balance As Double
    Dim initialBalance As Long
    Dim interest As Double
    Dim interestRate As Double
    Dim year As Long
 
    initialBalance = InputBox("Initial balance ($) ? ")
    interestRate = InputBox("Interest rate (%) ? ")
    balance = initialBalance
    
    ' Display column headings
    Debug.Print "Year", "Interest ($)", "Balance ($)"
    Debug.Print 0, , Format(balance, "#,0.00")

    ' Display table of values
    For year = 1 To 10
        interest = balance * interestRate / 100
        interest = Round(interest, 2) ' Round to 2 decimal places
        balance = balance + interest
        ' Format interest to 2 decimal places and right justify. Format balance to 2 decimal places
        Debug.Print year, Format(Format(interest, "0.00"), "@@@@@@@"), Format(balance, "#,0.00")
    Next

End Sub

2. Nested Control Structures

Calculations, assignment, Input, Display, IF, FOR, WHILE are known as statements. IF, FOR and WHILE are a special type of statement, known as control structures.

IF, FOR or WHILE statements can contain other statements including other IF, FOR or WHILE statements. A FOR loop inside a FOR loop is a called nested FOR loop.

Example 1: Menus

Menus are used to display a list of options from which the user can select one at a time and some appropriate action is performed. Simple menus can be implemented using a loop containing nested IFs.

The general logic for a menu is as follows:

menu()
    choice = ""
    WHILE menuChoice <> exitChoice
        Display menu
        Input choice
        IF menuChoice = choice1 THEN
            do something appropriate for choice 1
        ELSE
            IF menuChoice = choice2 THEN
                do something appropriate for choice 2
            ELSE
                IF menuChoice = choice-n THEN
                    do something appropriate for choice n
                ELSE
                    IF menuChoice <> exit-choice THEN
                        Display invalid option
                    ENDIF
                ENDIF
            ENDIF
        ENDIF
    ENDWHILE
STOP

Problem Description

Write a program that performs transactions on a bank account. The account balance if initially zero. The transactions are Balance (display the balance of the account), D (make a deposit to the account) and W (make a withdrawal from the account). X is used to indicate the user wishes to exit the program.

Example Program Run

Run

Transaction Menu
  B. Balance
  D. Deposit
  W. Withdraw
  X. Exit

Choice ? B

Balance = $0.00

Transaction Menu
  B. Balance
  D. Deposit
  W. Withdraw
  X. Exit

Choice ? D

Deposit amount ? 100

Transaction Menu
  B. Balance
  D. Deposit
  W. Withdraw
  X. Exit

Choice ? D

Deposit amount ? 50

Transaction Type
  B. Balance
  D. Deposit
  W. Withdraw
  X. Exit

Choice ? B

Balance = $150.00

Transaction Type
  B. Balance
  D. Deposit
  W. Withdraw
  X. Exit

Choice ? X

Defining Diagram

Inputs	Processing	Output
transType amount	Display transaction menu Input transType Input amount Perform transaction	balance

Outline the Solution

a nested IF is used for all of the menu options. The final ELSE is used to catch invalid values.

Pseudo Code

Data Dictionary

Name	Data Type	Description
amount	Double	The amount of the deposit or withdrawal
balance	Double	The current account balance in $
transType	String	The type of transaction. "B"=Balance, "D"=Deposit, "W"=Withdrawal, "X"=eXit

Example 2: Membership Fees

Problem Description

The following is a modification of a problem done in earlier lectures to calculate the membership fees for a club member.

The enhancements are:

validation of the membershipType
a loop so that the fees can be calculated for multiple members
a total of the membership fees is calculated and displayed

The logic has similarities to the banking program above. A point of difference is that the menu option is validated, so the nested IF statement does not need to allow for invalid values.

Defining Diagram

Inputs	Processing	Output
membershipType	Input membershipType Check membershipType is correct Determine the membershipFee Determine the membershipName Total the membershipFees Display membershipName, membershipFee Display totalFees	membershipName membershipFee totalFees

Outline the Solution

a While loop is to repeat the processing while the user doesn't input an "X"
a While loop is required to repeat asking for a membershipType while it is wrong.
a nested IF is required to determine the membershipFee and membershipName from the membershipType
a total is needed to sum the membershipFees

Pseudo Code

Discussion

Data Dictionary

Name	Data Type	Description
membershipFee	Double	The membership fee for a club member.
membershipName	String	The full name of the membership type: Full, Junior or Life
membershipType	String	The type of membership in the club. Values: F=Full, J=Junior, L=Life and X=Exit.
totalFees	Double	The total membership fees in $.

Desk Check

Access Basic Code

' Name: calcMembershipFees
' Purpose: Calculate the fees for club members
' Author: Tim Whitfort

Option Compare Database
Option Explicit
 
Sub Main()
 
   Dim membershipFee As Double  ' The membership fee for a club member.
   Dim membershipName As String ' The full name of the membership type: Full, Junior or Life
   Dim membershipType As String ' The type of membership in the club. Values: F=Full, J=Junior, L=Life and 
    X=Exit.
   Dim totalFees As Double ' The total membership fees in $.
 
   ' Initially the total fees are zero
   totalFees = 0
 
   ' Input and validate membershipType
   membershipType = InputBox("Membership Type (F=Full, J=Junior, L=Life, X=eXit) ?")
   While membershipType <> "F" And membershipType <> "J" And _
         membershipType <> "L" And membershipType <> "X"
      MsgBox ("Error: membership type must be F, J, X or X")
      membershipType = InputBox("Membership Type (F=Full, J=Junior, L=Life, X=eXit) ?")
   Wend
 
   ' While the user hasn't finished
   While membershipType <> "X"
 
      ' Determine the membershipFee and membershipName
      If membershipType = "F" Then
         membershipFee = 160
         membershipName = "Full"
      Else
         If membershipType = "J" Then
            membershipFee = 80
            membershipName = "Junior"
         Else
            membershipFee = 10
            membershipName = "Life"
         End If
      End If
 
      ' Sum the membership fees
      totalFees = totalFees + membershipFee
 
      ' Display the membership fee
      Debug.Print membershipName; " membership fee = $"; Format(membershipFee, "0.00")
 
      ' Input and validate membershipType
      membershipType = InputBox("Membership Type (F=Full, J=Junior, L=Life) ?")
      While membershipType <> "F" And membershipType <> "J" And _
            membershipType <> "L" And membershipType <> "X"
         MsgBox ("Error: membership type must be F, J, X or X")
         membershipType = InputBox("Membership Type (F=Full, J=Junior, L=Life, X=Exit) ?")
      Wend
 
   Wend
 
   Debug.Print "Total Fees = $"; Format(totalFees, "0.00")
 
End Sub

4. Review of Looping

Types of Loops

Repeat a fixed number of times (use a FOR loop)
Repeat a number of times that is input by the user (use a FOR loop)
Repeat until a sentinel value is input (use a WHILE loop)
Repeat until an ending condition is met that doesn't depend on a counter (use a WHILE loop)

Some Common Uses of Repetition

Printing tables of values
Inputting multiple values (then used in processing e.g. calculate tax)
Totalling
Averaging
Validating
Displaying and selecting from a menu

Approach to Solving Problems Involving Repetition

When solving problem that may involve repetition, determine:

Is there repetition
What sort of repetition is it (see Types of Loops above)
Is it a common type of problem (see Some Common Uses of Repetition above) if so use the standard logic for this type of problem
Is it a variation on some other problem you have seen - adapt the logic
What is repeated and what isn't?
- If something occurs once, then don't put it in the loop
- If something occurs once, then does it occur before the loop or after it?

Common Logic Errors

Having While loops where the condition never becomes False - Infinite loops
Forgetting to initialize a total to zero before a loop
Forgetting to input data at the end of a While loop that uses a sentinel value to determine if the user has finished inputting data
Repeating a loop one time to many or one time fewer than intended

Key Points

Displaying tables of values often involves incrementing one value in a loop, from which the rest of the values on the same line in the table can be calculated
IF, FOR and WHILE are control structures. They may be nested (place inside one another).

Lecture 14 - Repetition - Tables, Menus & Nested Control Structures

Objectives

Contents

1. Displaying Tables of Values

Problem Description

Example Program Run

Discussion

Defining Diagram

Outline the Solution

Pseudo Code

Data Dictionary

Desk Check

Access Basic Code

2. Nested Control Structures

Example 1: Menus

Problem Description

Example Program Run

Defining Diagram

Outline the Solution

Pseudo Code

Data Dictionary

Example 2: Membership Fees

Problem Description

Defining Diagram

Outline the Solution

Pseudo Code

Data Dictionary

Desk Check

Access Basic Code

4. Review of Looping

Types of Loops

Some Common Uses of Repetition

Approach to Solving Problems Involving Repetition

Common Logic Errors

Key Points

Further Reading